Ventilator-dependent premature infants are often treated with dexamethasone. However, the optimal timing of therapy is unknown. We compared the benefits and hazards of initiating dexamethasone therapy at two weeks of age and at four weeks of age in 371 ventilator-dependent very-low-birth-weight infants (501 to 1500 grams) who had respiratory-index scores (mean airway pressure x the fraction of inspired oxygen) of greater than or equal 2.4 at two weeks of age. The primary outcome was the number of days from randomization to extubation not requiring reintubation (extubation score or death). The secondary outcomes were death before discharge from the hospital; the duration of assisted ventilation, supplementary oxygen therapy and hospital stay; the incidence of chronic lung disease (defined as the need for supplemental oxygen at 36 weeks postconceptional age by best obstetrical estimate) and rates of morbidity and mortality from respiratory causes during the first year. Additional secondary endpoints were hyperglycemia, hypertension, growth, bacteremia, necrotizing enterocolitis and upper GI bleeding.